Parting blades of the type generally mentioned above are mountable in fixed holders in order to, together with a wear part in the form of a replaceable turning insert, form a turning tool having the purpose of carrying out parting or grooving operations in rotating workpieces. Typically, the blade—at least near the insert seat—has to have a thickness that is smaller than the width of the turning insert carried by the blade, because otherwise the blade would not go free in the gap generated in connection with the grooving of the turning insert into the workpiece. The so-called insert-tip height has a great importance to the dimensioning of the tools, which height is a measure of the position or level of the insert seat in relation to the lower longitudinal edge of the blade. It is always desirable that the insert seat is located as high as possible in the overhung end of the blade that protrudes from the tool holder, in order to, in such a way, endow the front, supporting blade portion that is present under the insert seat with the greatest possible stiffness. Namely, if this portion would become too weak, the same runs the risk of being deflected or deformed by the very great forces that are applied to the turning insert during machining.
Tools of different dimensions are provided for different applications. Thus, on the market an assortment of small tools is offered, which have an insert-tip height of 21 mm at the same time as the appurtenant turning inserts have a width that varies within the range of 1.5-3 mm. Another assortment—in the form of medium-sized tools—has an insert-tip height of 25 mm and a set of turning inserts the width of which varies within the range of 1.5-8 mm. Furthermore, tools that are even more robust may also be found. Within the different assortments, different solutions are found to the problem of reliably fixing the turning insert in the insert seat. A usually occurring solution is the so-called spring clamping. In this case, the turning insert is kept clamped only by means of the inherent elasticity or flexibility of a clamping finger present above the bottom of the insert seat. Another solution is screw clamping. In this case, the clamping finger is actuated also by a screw, which upon tightening, clamps the finger by a force that is considerably greater than the plain spring-tension force. In other words, screw clamping is more reliable than spring clamping.
The holder used to clamp the blade and which may be included in a turning lathe or a multioperation machine is a robust block, which includes side support surfaces against which an inside of the blade may be pressed, and which has a lower, fixed jaw against which the lower longitudinal edge of the blade is applied. A movable upper jaw co-operates with a number of screws, which upon tightening press the movable jaw against the upper longitudinal edge of the blade. The two longitudinal edges of the blade are formed with chamfered surfaces, which converge in the direction outward and co-operate with analogous, although inwardly converging chamfer surfaces of the jaws. Therefore, when the jaws approach each other by the tightening of the screws, a wedge action arises, which by a great force presses the inside of the blade firmly against the side support surfaces of the block.
Because different types of blades have insert-tip heights of different sizes, one type of holder is required for each type of blade. In practice, the machine equipment of different workshops may vary most considerably. For instance, certain workshops may just have a single type of holder, while others have holders for small as well as large blades. A problem being common to all users—irrespective of the machine equipment—is, however, that the small blades (insert-tip height of 21 mm) only are suitable for the very smallest turning inserts (insert width of max 3 mm). Namely, if a larger and wider turning insert is attempted to be mounted on the smallest blade, the flexural rigidity of the front, supporting end portion of the blade would become too small to be able to resist the cutting forces acting on the turning insert. The section in the area under the insert seat would simply become too weak, since the amount of material (steel) in this section is limited. Naturally, it is feasible to mount the larger turning insert on a more robust blade (insert-tip height of 25 mm), but in such a case, the users only having access to holders for small blades would need to acquire larger holders and maybe rebuild existing machines.